1. Field of the Invention
The present invention relates to a sintered zirconia material and a method for manufacturing the material.
2. Description of the Prior Art
There are several kinds of sintered zirconia materials. JP-A-57-111278 discloses, so called, a high strength sintered zirconia material. This conventional sintered zirconia material is a material containing 5-70 mol% zirconia having a tetragonal crystal structure (tetragonal zirconia) and the porosity of the material is 2-10%. In the sintered material, since the tetragonal zirconia is transformed to a zirconia having a monoclinic crystal structure (monoclinic zirconia) when receiving thermal shock and thereby expanding by about 4 vol. %, compressive stress fields are formed in the monoclinic zirconia or near the monoclinic zirconia. Since the compressive stress field acts so as to absorb strain due to the thermal shock, the strength against thermal shock of the sintered material is high. Also, since the compressive stress field acts so as to decrease an elastic strain energy when the material receives external stress, the bend strength of the material increases.
However, the increase in the bend strength is not so remarkable because the porosity is relatively high that is, 2-10%, and the deviation of the strength is large. Moreover, there is the defect that the toughness of the material is low.
On the other hand, JP-A-59-227770 discloses a sintered zirconia material obtained by sintering a zirconia powder containing a stabilizer and a black colorant in an inert atmosphere by hot pressing or hot isostatic pressing, using a graphite mold or die. It is described in the specification that the sintered material as obtained above has high heat resistance and high fracture toughness.
However, since the graphite mold is used and the material is sintered in an inert atmosphere, carbon remains in the material. Since the carbon evaporates by becoming carbon dioxide gas and a cavity is formed in the position where the carbon was positioned, when the material is used under temperature conditions of higher than 600.degree. C., the strength of the material decreases greatly. Also, the strength of the material decreases even at room temperature.